Digital communications is critical to the success of modern communications technology. Orthogonal frequency division multiplexing (OFDM) is a viable modulation scheme for packet communications. It has been adopted in a number of communication standards including DSL, 802.11 WiFi, 802.16 WiMAX, 802.22 WRAN, and 3GPP LTE etc. One of the major limitations of OFDM is frequency synchronization errors caused by frequency differences between a local oscillator in a transmitter and the local oscillator in a receiver of an OFDM modem. This frequency difference is called carrier frequency offset (CFO). CFO contributes to a number of impairments to OFDM system performance, such as inter carrier interference (ICI) among OFDM subcarriers, attenuation and constellation rotation. Needless to say, one important signal processing function in an OFDM receiver is to estimate and remove CFO.
In an OFDM system, a transmitted data sequence usually includes a training sequence (preamble) and predefined pilot subcarriers in each OFDM symbol. For example, in the proposed 802.22 standard, a superframe preamble is transmitted at the beginning of each superframe, and a frame preamble is transmitted at the beginning of each frame. Each superframe contains 16 frames and each frame contains 25 to 41 OFDM symbols and has a duration of 10 ms. Moreover, in each OFDM symbol 1680 subcarriers are used, and 240 of those subcarriers are pilot subcarriers. The value transmitted on the pilot subcarriers is predetermined and known to the receiver.
Classical CFO estimation methods examine information in the OFDM preambles to detect and estimate CFO. However, channel imperfections and noise deteriorate CFO estimation accuracy, and residual CFO remains in the received data sequence after CFO estimation and removal has been performed. Since there are usually a large number of OFDM data symbols between two preambles, the residual CFO may result in significant performance degradation.
The detection of residual CFO in the frequency domain is known, as taught in U.S. Pat. No. 7,526,020 which issued on Apr. 28, 2009 to Kao et al.